One of the basic operational principle of liquid crystal displays and devices is the switching of the orientation of the liquid crystal molecules by an applied electric field that couples to the dielectric anisotropy of the liquid crystal material (dielectric coupling). Such a coupling gives rise to an electro-optic response quadratic with the applied electric field, i.e. independent of the field polarity.
The dielectric anisotropy (Δ∈) of a material having an ordered molecular structure (ordered phase) possessing a structural anisotropy, such as a crystalline or a liquid crystalline structure, is the difference between the dielectric constants measured in perpendicular and parallel direction, respectively, to the preferred molecular orientation in this material.
When an electric field is applied across a liquid crystal material exhibiting a positive dielectric anisotropy (Δ∈>0), the molecules will align their long axis along (or substantially along) the direction of the electric field.
When an electric field is applied across a liquid crystal material exhibiting a negative dielectric anisotropy (Δ∈<0), the molecules will align their long axis perpendicular (or substantially perpendicular) to the direction of the electric field.
Liquid crystal molecules are long rod-like molecules (so-called calamitic molecules) which have the ability to align along their long axis in a certain preferred direction (orientation). The average direction of the molecules is specified by a vector quantity and is called director.
In liquid crystal displays, the desired initial alignment of the liquid crystal layer in the absence of an external field, such as an electric field, is generally achieved by appropriate surface treatment of the confining solid substrate surfaces, such as by applying a so-called (surface-director) alignment layer (orientation layer) on the confining substrate surfaces facing said liquid crystal bulk. The initial liquid crystal alignment is defined by solid surface/liquid crystal interactions. The orientation of the liquid crystal molecules adjacent to the confining surface is transferred to the liquid crystal molecules in the bulk via elastic forces, thus imposing essentially the same alignment to all liquid crystal bulk molecules.
The director of the liquid crystal molecules near the confining substrate surfaces (herein also called surface director) is constrained to point in a certain direction, such as perpendicular to (also referred to as homeotropic or vertical) or in parallel with (also referred to as planar) the confining substrate surfaces. The type of alignment in liquid crystal displays operating on the coupling between liquid crystal dielectric anisotropy and applied electric field is chosen in accordance with the sign of the dielectric anisotropy, the direction of the applied electric field and the desired type of switching mode (in-plane or out-of-plane).
In out-of-plane switching liquid crystal cells employing a liquid crystal bulk having a negative dielectric anisotropy, it is important to uniformly orient the director of the liquid crystal bulk molecules (in the field-off state) vertically to the substrate surfaces (so-called homeotropic alignment).
An example of a method for establishing a homeotropic alignment comprises coating the confining substrate surfaces with a surfactant, such as lecithin or hexadecyltrimethyl ammonium bromide. The coated substrate surfaces is then also preferably rubbed in a predetermined direction, so that the field-induced planar alignment of the liquid crystal molecules will be oriented in the predetermined rubbing direction. This method may give good results in laboratory studies, but has never found industrial acceptance due to that long term stability is not obtained as the alignment layer is slowly dissolved in the bulk liquid crystal (J. Cognard, Mol. Cryst. Liq. Cryst., Suppl. Ser., 1982, 1, 1).
In out-of-plane switching liquid crystal cells employing a liquid crystal bulk having a positive dielectric anisotropy and in in-plane switching liquid crystal cells employing a liquid crystal bulk having a positive or negative dielectric anisotropy, it is important to uniformly orient the director of the liquid crystal bulk molecules (in the field-off state) in parallel with the substrate surfaces (so-called planar alignment). For twisted nematic liquid crystal cells, it is also important to orient the liquid crystal bulk molecules at a certain inclined orientation angle (pre-tilt angle) to the substrate.
Known methods for establishing planar alignment are, for instance, the inorganic film vapour deposition method and the organic film rubbing method.
In the inorganic film vapour deposition method, an inorganic film is formed on a substrate surface by vapour-deposition of an inorganic substance, such as silicon oxide, obliquely to the confining substrate so that the liquid crystal molecules are oriented by the inorganic film in a certain direction depending on the inorganic material and evaporation conditions. Since the production cost is high, and the method thus is not suitable for large-scale production, this method is practically not used.
According to the organic film rubbing method, an organic coating of, for instance, polyvinyl alcohol, polyoxyethylene, polyamide or polyimide, is formed on a substrate surface. The organic coating is thereafter rubbed in a predetermined direction using a cloth of e.g. cotton, nylon or polyester, so that the liquid crystal molecules in contact with the layer will be oriented in the rubbing direction.
Polyvinyl alcohols (PVA) are commercially rarely used as alignment layers since these polymers are hydrophilic, hygroscopic polymers that may adsorb moisture adversely affecting the molecular orientation of the polymer and thus the liquid crystal device performance. In addition, PVA may attract ions which also impairs the liquid crystal device performance.
Also polyoxyethylenes may attract ions, thus resulting in impaired liquid crystal device performance.
Polyamides have a low solubility in most commonly accepted solvents. Therefore, polyamides are seldom used commercially in liquid crystal device manufacturing.
Polyimides are in most cases used as organic surface coating due to their comparatively advantageous characteristics, such as chemical stability, thermal stability, etc. The application of a polyimide layer generally includes a baking step at 200-300° C. as described below.
Polyimides may be prepared according to, for instance, Scheme I or Scheme II below:


In the first step, equimolar amounts of a tetracarboxylic acid anhydride and a diamine are mixed in an amide solvent, such as N-methylpyrrolidone (NMP). A spontaneous reaction occurs and a polyamic acid, which is a pre-polymer of polyimide, is formed. In this state, the pre-polymer is distributed to its users, such as LCD manufacturers. However, since the pre-polymer solution is rather unstable at room temperature, the solution is generally cooled upon transportation and storage to avoid degradation, or any other unwanted chemical reaction, of the pre-polymer.
Generally, the polyamic acid is diluted by the liquid crystal device manufacturer to about 0.5%, often with a mixture of NMP and Butyl Cellosolve 4:1 (w/w).

The polyamic acid is generally applied using, for instance, spin coating or some type of printing technique on a glass substrate coated with a transparent, patterned indium tin oxide (ITO) electrode layer. The layer of polyamic acid is then dried in an oven at around 100° C., and thereafter heated to about 200° C. for 1-2 h. During this heating cycle polyamic acid is converted to polyimide. This step is also referred to as curing or baking of the polyimide. The resulting polyimide is thermally very stabile and insoluble in all solvents. The polymer can only be removed by degrading it, for instance, using an alkaline medium.
A drawback of this organic film application process is the baking step, resulting in both a long production time and high production costs.
Furthermore, high temperatures, such as about 200° C., should be avoided in the manufacturing of, for instance, liquid-crystal-on-silicon (LCOS) and thin film transistors (TFT) since high temperatures may result in decreased yields and thus film defects.
It is also difficult to control the anchoring strength between the organic film applied using said organic film application process and a liquid crystal bulk layer.
It would be a great advantage if said baking step could be eliminated and the above disadvantages avoided.